(a) Field of the invention
This invention relates to an epidark illumination device whereby, in observing an object to be inspected having a light transmissibility but including impurities or optically anisotropic substances with such observing optical system as a stereomicroscope, a dark background part is made on the object by preventing an illuminating light from directly entering the observing optical system so that only the light diffused by the object of the illuminating light can be observed.
(b) Description of the prior art
There is known a conventional epidark illumination device of this kind whereby, as mentioned, for example, in Japanese Patent Laid-Open No. 140811/1980, a light from a light source is made annular by a light guiding member made of known glass fibers and the light coming out of the exit end face of the guiding member is reflected by an annular reflecting face to illuminate an object to be inspected. As shown in FIG. 1, this is formed of a light source 1, a light guiding member 2 made of a known glass fiber bundle and having the entrance end 2a near the light source 1 formed to be bar-shaped and the exit end 2b formed to be annular and a reflector 3 arranged concentrically on the exit end 2b and having a conical reflecting face 3a so that the light emitted from the light source 1 will be transmitted through the light guiding member 2 to become an annular light and then will be reflected by the conical reflecting face to be collected in the direction of an object 4 to be inspected. In order to minimize the reflection of the ground surface of the object 4 to be inspected, the conical reflecting face 3a has been formed of a rough surface so that the reflected light may be diffused. There has been also an example in which, in order to increase this diffusing effect, as shown in FIG. 2, a glass mat 5 is placed on the reflector 3.
However, there has been a problem that, in the example shown in FIG. 1, in order to effectively illuminate the object 4 to be inspected, it is necessary to set the angle of inclination of the conical reflecting face 3a so that most of the light coming out of the exit end face 2b may enter the object 4 but, if the angle of inclination of the conical reflecting face 3a is so set, as the reflecting face 3a is of a rough surface, a part of the diffused illuminating light will directly enter the observing optical system and the epidark effect will be greatly reduced. There has been also a problem that, if the conical reflecting face 3a is formed of a mirror surface, the annular illuminating light will be collected in the central part by the action of the reflector 3 so as to form a bright spot and an unflat illumination will be likely to be made.
There has been also a problem that, even in the example shown in FIG. 2, a part of the illuminating light reflected by the conical reflecting face 3a and diffused by the glass mat 5 will be likely to enter the observing optical system to reduce the epidark effect.
Therefore, in order to interrupt such stray light as is caused by diffusion, as mentioned, for example, in Japanese Utility Model Publication No. 11051/1970, it has been invented to provide a light interrupting cylinder which will not interrupt the illuminating light that will interrupt only the stray light above a frosted plate bearing frame in a dark illumination device wherein a light source is arranged in the center of a reflector, a frosted plate (frosted glass plate) is provided just above the light source and an iris diaphragm is provided near window for illuminating the object to be inspected. As shown in FIG. 3, this is formed of a housing 10, a cylinder body 11 fixed to the housing 10 and having reflecting mirror surface 11a, a light source 13 positioned in the central part of the reflecting mirror surface 11a and fixed to the bottom of the cylinder body 11 through a supporting frame 12, a bearing frame 15 positioned just above the light source 13 and fixed to the cylinder body 11 through an arm 14, a frosted plate 16 fixed to the bearing frame 15, a shutter plate 17 provided just above the frosted plate 16 so as to be free to open and close and opened and closed with a shutter lever 17a projected out of the cylinder body 11, a light interrupting cylinder 18 for preventing stray lights formed integrally with the bearing frame 15 so as to extend above the bearing frame 15, a lid 19 fixed to the top part of the housing 10 and having an opening 19a concentric with the center axis O of the light path, an iris diaphragm 20 arranged just below the opening 19a and operated with a handle 20a, a bowl-shaped frosted plate 21 positioned just below the iris diaphragm 20, fitting to the light interrupting cylinder 18 and fixed to the lid 19, a supporting rod 23 fixed on the lid 19 and having a holder 23a for holding the object 22 to be inspected and a connecting bar 24 connecting the entire illumination device with a microscope body not illustrated. By the way, the curved surface of the above mentioned bowl-shaped frosted plate 21 is made such surface on which the light bundle reflected by the reflecting mirror surface 11a will be incident substantially vertically to reduce the light amount reflected by said surface to be as small as possible. Therefore, if the frost of the bowl-shaped frosted plate 21 is of fine grains, the direction of the light having been transmitted through the bowl-shaped frosted plate 21 will be substantially kept. The reference numeral 25 denotes an objective of the observing optical system.
The light emitted from the light source 13 will be reflected by the reflecting mirror surface 11a and will be substantially vertically incident on the surface of the bowl-shaped frosted plate 21 to illuminate the object 22 to be inspected, only the light diffused by the object will enter the objective 25 and therefore only the object has been observed as brightly floated up in the dark visual field. In this case, needless to say, as the shutter plate 17 is closed, the light just above the light source 13 will be interrupted. Also, the stray light produced when the light bundle led by the reflecting mirror surface 11a or the light transmitted through the bowl-shaped frosted plate 21 is diffused by the surrounding wall plate or the like will be interrupted by the light interrupting cylinder 18.
However, there has been a problem that, in the case of this example, as the off-axis illumination is prevented by transmitting the illuminating light through the bowl-shaped frosted plate and the stray light produced when the illuminating light is transmitted through the bowl-shaped frosted plate is interrupted by the light interrupting cylinder 18, because of setting the light interrupting cylinder 18 or the like, the entire illuminating device will be complicated in the structure and will be costly and the loss of the light by interrupting the light will be large.